This invention relates to coupled transmission lines for electromagnetic waves and, in particular to coupled transmission lines that are tunable and/or tolerant of manufacturing variations.
Coupled transmission lines are used to make a variety of high frequency electromagnetic wave devices (RF and microwave) including frequency selective filters, signal splitters, and combiners, and delay lines. A typical coupled transmission line comprises a pair of elongated conductive strips separated by an intervening layer of dielectric material. FIG. 1, which is prior art, illustrates a transverse cross section of a common form of a coupled transmission line 10 wherein a pair of conductive strips 11A and 11B are separated by a gap of spacing G on a dielectric substrate 12 having a dielectric constant E. This form of the transmission line where the strips 11A and 11B overlap across at least a portion of the gap is referred to as a broadside coupled line.
FIG. 2, also conventional, illustrates an alternative coupled transmission line 20 wherein the conductive strips 11A and 11B are partially offset across the gap G. The amount overlap between 11A and 11B is designated L. This type of transmission line is called an off-set broadside coupled transmission line.
In each form, the degree of coupling C between the two strips 11A and 11B is a key parameter in the function of the transmission lines and devices using them. C is inversely proportional to the gap spacing G, and jointly proportional to the square root of the dielectric constant E and the overlap L.
In the fabrication of coupled transmission lines, it is difficult to control with desired precision the degree of coupling C. Common methods for manufacturing broadside coupled lines include thin film and thick film circuit technology, laminated printed circuit board technology, low temperature cofired ceramic (LTCC) technology and high temperature cofired ceramic (HTCC) technology. In these technologies, the degree of coupling is affected by manufacturing variations in dielectric constant, conductor width, conductor-to-conductor misalignment and dielectric thickness. Accordingly, there is a need for a broadside coupled transmission line structure that is tolerant of normal manufacturing variation and/or can be readily tuned.
In accordance with the invention, coupled transmission lines are fabricated in forms that are tolerant of manufacturing variation or can be readily tuned. In accordance with a first embodiment, the transmission line is formed with alternating overlapping edges for enhanced manufacturing tolerance. In a second embodiment, the line is provided one or more overlapping adjustment regions to permit tuning. A third embodiment has both alternating overlapping edges and one or more tuning regions.